The production of multicomponent thermoplastic fibers and filaments has been known in the art for some time. The term “multicomponent” generally refers to fibers that have been formed from at least two polymer streams which have been brought together to form a single, unitary fiber. Typically the separate polymer streams are brought together just prior to or immediately after extrusion of the molten polymer to form filaments. The polymer streams are brought together and each forms a distinct component arranged in substantially constantly positioned distinct zones across the cross-section of the fiber. In addition, the distinct components also extend substantially continuously along the length of the fiber. The configuration of such fibers can vary and commonly the individual components of the fiber are positioned in a side-by-side arrangement, in a sheath/core arrangement, in a pie arrangement, an islands-in-sea arrangement or other configuration. As but a few examples, multicomponent filaments and methods of making the same are described in U.S. Pat. No. 5,108,820 to Kaneko et al., U.S. Pat. No. 5,382,400 to Pike et al., U.S. Pat. No. 5,277,976 to Hogle et al., U.S. Pat. No. 5,466,410 to Hills and U.S. Pat. Nos. 3,423,266 and 3,595,731 both to Davies et al. Multicomponent fibers offer various advantages such as the ability to form fabrics having fiber crimp, autogenous bonding, good hand and/or other desirable characteristics. Thus, multicomponent spunbond fibers have found useful applications, both alone and in laminate structures, in personal care articles, filter materials, industrial and personal wipers, medical fabrics, protective fabrics and so forth.
Typically, the multicomponent fibers are made from two different polymers such as, for example, polypropylene and polyethylene, polyethylene and nylon, polyethylene and PET and so forth. As described in U.S. Pat. No. 5,382,400 to Pike et al., by Employing polymers having considerably different melting points it is possible to bond the fabrics made therefrom by through-air bonding. The low melting component can be sufficiently heated so as to form bonds at fiber contact points whereas the high melting component retains the integrity of both the fiber structure and the structure of the fabric. The differences in melting points can also be used to form a helical crimp within the multicomponent fibers. As a further example, U.S. Pat. No. 4,323,626 to Kunimune et al. teaches fine multicomponent fibers having a thin adhesive component having a uniform thickness. The fibers of Kunimune comprise a first polypropylene component having a melt-flow rate between 1-50 g/10 minutes and a second ethylene-vinyl acetate component having a melt-index of 1-50 g/10 minutes. The second component comprises a portion of the outer surface of the fibers and can have a higher melt-index than the melt-flow rate of the first polypropylene component. However, Kunimune et al. teaches that use of the second component should not vary outside the melt-index range of 1-50 g/10 minutes since decomposition during the spinning process otherwise occurs. As taught in Kunimune, conventional practice has been to utilize polymeric components with similar melt-flow rates. Additionally, conventional practice also typically employs polymers having lower melt-flow rates since utilization of polymers with higher melt-flow rates or disparate melt-flow rates can often cause filaments to break or otherwise decompose during melt-attenuation steps.
However, relatively higher melt-flow rate polymers have been successfully utilized heretofore in spinning fine denier thermoplastic polymer fibers. U.S. Pat. No. 5,681,646 to Ofosu et al. teaches that high melt-flow rate polymers, such as polypropylene having a MFR of between about 50 and 150 g/10 minutes, can be used to make high strength fibers. In addition, use of such high melt-flow rate polymers is also taught in U.S. Pat. No. 5,672,415 to Sawyer et al. More particularly, Sawyer et al. teaches a multicomponent fiber having a first ethylene polymer component having a melt-index between 60-400 g/10 minutes and a second propylene polymer component having a melt-flow rate between 50-800 g/10 minutes. Use of the relatively high melt-flow rate polymers provides fine fibers, enhances crimp and also improves certain aspects of the spinning process. However, while relatively higher melt-flow rate polymers are taught in Sawyer et al., the examples of Sawyer et al. employ polymeric components having relatively similar melt-flow rates. Use of disparate melt-flow rates would be expected to create problems in the spinning and/or melt-attenuation steps such as, for example, fiber breakage.
An increasing variety of high melt-flow rate polymers are being developed as a result of current improvements in polymerization processes and catalysts. Notably, the use of metallocene and/or constrained geometry catalysts used in the production of olefin polymers has provided an ever increasing variety of polymers with distinct physical and/or rheological properties. In particular, high melt-flow rate polymers suitable for spinning are becoming more widely available. However, fiber production processes that require a melt-attenuation step as a means for molecularly orienting the polymer and/or reducing the fiber diameter have an inherent limitation with regard to the usefulness of such high melt-flow rate polymers. As the melt-flow rate increases, the amount of attenuating force that may be applied to the molten filament decreases since the higher melt-flow rate polymers have a lower melt viscosity and are therefore more prone to break at lower attenuating forces. Thus, there exists a need for methods of producing fibers that are capable of utilizing high melt-flow rate polymers and further which are capable of adequately melt-attenuating the same.